Conducting materials such as superconducting materials and transparent conducting materials have a wide range of applications. For example, transparent conducting oxide (TCO) films are useful for optical display devices such as cell phones, personal digital assistants, flat panel displays, plasma screens, computers, and various touch panel devices, e.g., ATM machines. TCOs are also used in photovoltaics, such as thin-film solar cells, liquid crystal displays, light-emitting diodes, and as antistatic and antifogging coatings and EMI shieldings.
Several methods of depositing films on substrates are known, including sputtering techniques, chemical vapor deposition (CVD), spray pyrolysis, combustion chemical vapor deposition (CCVD), and pulsed laser deposition. In some instances, the films that are produced by these methods are conducting and/or transparent.
The most commonly employed technique to deposit crystalline thin films onto a cool substrate is sputtering. Although different films require different sputtering conditions, a variety of techniques for depositing transparent conductive oxides, including Tin-doped Indium Oxide (ITO) and Aluminum-doped Zinc Oxide (AZO or ZnO:Al), have been disclosed. See, for example, U.S. Pat. Nos. 7,309,405; 5,458,753; Japanese Examined Patent Publication No. 72011/1991; Minami, Nanto, & Takata “Highly conductive and transparent ZnO thin films prepared by RF magnetron sputtering in an applied external DC magnetic-field”, Thin Solid Films, 1985, 124, 43-47. However, the technique of sputter-coating a substrate almost universally requires a sealed vacuum chamber and as such is implemented as a batch process. It is therefore not conducive to large-scale production.
CVD methods also typically require a low-pressure or sealed chamber, thus making these techniques less favorable for large scale production of thin films. However, Hu & Gordon reported producing AZO films under atmospheric-pressure vapor deposition of a heated mixture of diethyl zinc, triethyl aluminum, and ethanol. Hu & Gordon, “Textured aluminum-doped zinc oxide thin-films from atmospheric-pressure chemical-vapor deposition” J. of App. Phys., 1992, 71, 880-890. Nevertheless, high substrate temperatures between 367° C. and 444° C. were recorded for satisfactory results. The high substrate temperatures are believed to allow surface rearrangement reactions to occur to promote reasonable crystallinity of the deposited film.
Spray pyrolysis and CCVD both involve the formation by combustion of the molecular precursors or molecular clusters to be deposited into a crystalline film, and both techniques may be used at atmospheric pressures to form thin films. Both, however, typically require substrate temperatures of roughly 450° C. Kaid & Ashour (“Preparation of ZnO-doped Al by spray pyrolysis technique”, Applied Surface Science, 2007, 253, 3029-3033) demonstrate the fabrication of AZO film by spray pyrolysis of ammonium nitrate and zinc acetate onto a stationary substrate heated to an optimal temperature of 450° C. U.S. Pat. No. 6,013,318 discloses the application of CCVD to the fabrication of thin-films, whereby combustion of a fuel, oxidizer, and precursor gases or liquids results in vapor-phase compounds which deposit onto a nearby substrate. This disclosure relates to diffusion flames, i.e., flames that are formed when a fuel and oxidizer are injected separately and meet at about the flame front. Further, Zhao et al (“Transparent Conducting ZnO:Al Films via CCVD for Amorphous Silicon Solar Cells”, preprint, 29th IEEE PVSC, New Orleans, May 2002) disclosed an open-air CCVD process in which ZnO:Al films were deposited onto hot borosilicate glass substrates over a flame. The metal precursors were delivered into the flame by a flow of oxygen.
Another disclosed technique aimed at keeping the substrate cool during deposition is pulsed laser deposition (U.S. Pat. No. 6,818,924), whereby a substrate is placed in the vicinity of a bulk material, which is subsequently irradiated with a pulsed laser. Material is evaporated from the target onto the substrate, keeping the substrate cool. This technique resembles sputtering, in that material to be deposited needs to be evaporated in a reduced atmosphere.
Processes for preparing particle coatings, i.e., titanium dioxide particle coatings, by a stagnation flame have been reported (see, for example, McCormick et al., “Thermal Stability of Flame Synthesized Anatase TiO2 Nanoparticles”, J. Phys. Chem. B, 2004, 108, 17398-17402; and Zhao et al., “Ultrafine Anatase TiO2 Nanoparticles Produced in Premixed Ethylene Stagnation Flame at 1 Atmosphere”, Proc. Combustion Institute, 2005, 30, 2569-2576). These articles describe forming a stagnation flame a short distance below a stationary, stabilizing plate and collecting titanium dioxide particles on to a substrate that is translated into and out of the flame. These articles do not disclose producing a stagnation flame against a mobile stabilizing plate.
Despite the disclosure of forming conducting materials on substrates, it is desirable to provide a process that can form conducting materials on moving substrates to improve throughput and manufacturing yield. It is further desirable to provide processes that can form conducting materials on substrates in a continuous process under mild conditions and low substrate temperatures.